Subtopic: Fundamentals of Cell structure and signaling
Question Statement:
Which one of the following statements is true for actin?
Options:
(A) Actin filament is structurally polarized and the two ends are not identical
(B) De novo actin polymerization is a single-step process
(C) The pointed end of the actin filaments is the fast growing end
(D) Actin forms spindle fibers during mitosis
Official Answer Key:
A
Step-by-Step Solution & Explanation
### **Official Solution & Step-by-Step Derivation (GATE BT 2018 Q.25)**
**1. Governing Concept & Question Classification**:
- **Section**: BT
- **Question Type**: **MCQ** (1 Mark)
- **Topic**: Cell Biology (Cytoskeleton & Actin Microfilament Dynamics)
**2. Detailed Step-by-Step Analysis**:
- Actin microfilaments are polymers of globular actin (G-actin) subunits that assemble head-to-tail to form filamentous actin (F-actin).
- Because all asymmetric G-actin monomers point in the same direction along the filament axis, the actin filament possesses **structural polarity**; the two ends are distinct:
- **Barbed end (+ end)**: Characterized by fast assembly (high association rate constant $k_{ ext{on}}$).
- **Pointed end (- end)**: Characterized by slow assembly/disassembly (lower $k_{ ext{on}}$).
- Hence, the statement that *"Actin filament is structurally polarized and the two ends are not identical"* is completely **TRUE**.
- **Option-by-Option Evaluation**:
- **Option (A) [CORRECT]**: Actin filaments exhibit structural and kinetic polarity with non-identical plus (barbed) and minus (pointed) ends.
- **Option (B) [INCORRECT]**: *De novo* actin polymerization is a multi-step nucleation-elongation process: a rate-limiting nucleation step (trimer formation) followed by rapid elongation.
- **Option (C) [INCORRECT]**: The *barbed* end (+ end) is the fast-growing end, not the pointed end.
- **Option (D) [INCORRECT]**: Mitotic spindle fibers are composed of *microtubules* ($lpha/eta$-tubulin heterodimers), not actin filaments.
**3. Final Answer / Verification Summary**:
- **Official GATE Answer Key**: **A**
- **Scoring Criteria**: +1.0 Mark (Negative marking: -0.33 marks).
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